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Given the following: $\ce{CN-,OH-,SH-,I-}$. Determine the relative order of nucleophilicity in a protic solvent.

I know that, more the nucleophile is solvated by hydrogen bonding in a protic solvent, the lesser will be its nucleophilicity. Also, solvation depends upon the size of the anionic nucleophile or its charge density (smaller the size greater the solvation).

Again larger the size, greater will be the polarizability of the anionic nucleophile and thus stronger will be its nucleophilicity.

All these indicate that the larger the size, the greater the nucleophilicity will be, and so the relative order of nucleophilicity should be $\ce{I-}>\ce{SH-}>\ce{CN-}>\ce{OH-}$. But the answer is $\ce{SH-}>\ce{CN-}>\ce{I-}>\ce{OH-}$. So where have I gone wrong? How can I determine nucleophilicity accurately? Is there any other factor on which nucleophilicity depends?

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It has been the lifetime research project of Professor i.R. Mayr of the University of Munich (LMU) to determine reactivity scales of nucleophiles. They key points are that any nucleophilic addition’s rate constant can be derived by the equation

$$\lg k_\pu{20^\circ C} = s_\mathrm{N}(N + E)\tag{1}$$

where $E$ is an electrophilicity parameter, $N$ is a nucleophilicity parameter and $s_\mathrm{N}$ is a solvent and nucleophile-dependent additional nucleophilicity factor. A multitude of nucleophiles and electrophiles have been entered into this scheme over the years and a few posters showing nucleophilicity scales have been created (see the webpage).

Mayr’s research could show that nucleophilicity depends on the solvent and on the nucleophile, that it is a kinetic parameter and that it can only be estimated to a certain extent. Simple parameters such as size or even the charge per volume ratio are never enough to explain a nucleophile’s relative nucleophilicity unless all other parameters remain the same.

So your discussion which seems to base itself on size alone is bound to fail.

I wholeheartedly invite you to scrutinise the database data (also linked on the webpage) to attempt to find truly general rules. If you do, do not hesitate to submit them to the academic journal of your choice.

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  • $\begingroup$ I understand. But still is there any explanation for the nucleophilicity order of my problem? $\endgroup$ – Pranoy De Mar 21 '17 at 19:16

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